1. Field of the Invention
The present invention is directed to a loop antenna device for generating a magnetic field. The loop antenna device is adapted to be disposed as an antenna in the vicinity of a conductor made of metal.
2. Related Art
One of the conventional loop antenna devices is disclosed in German Patent Publication DE 41 05 826 A1. The conventional loop antenna device includes a first antenna A1 and a second antenna A2. The first antenna A1 has a coil L2 wound around a ferrite rod and a resonant capacitor C2 connected thereto in parallel which constitutes a parallel resonant circuit. The second antenna A2 has a circular coil L1 accommodating therein the ferrite bar B and a resonant capacitor C1 connected in parallel with the circular coil L1 which constitutes a parallel resonant circuit. The ferrite rod 52 is also wound with coil L3 to which an amount of current is fed from a power source S.
In the foregoing structure, the ferrite rod 52 is rotated through an angle so as to establish a magnetic coupling between the first antenna A1 and the second antenna A2.
FIG. 6(b) of the present application shows an equivalent circuit of a conventional structure as shown in FIG. 6(a). In this case, when the loop antenna device is oscillated by a power supply S, a magnetic field component Hz is generated by the coil L1 and makes an angle of 90 degrees relative to a magnetic field component Hy generated by the coil L2. It is to be noted that the magnetic field component Hz and the magnetic field component Hy extend in the z-direction and y-direction, respectively.
For example, when the loop antenna device 51 is part of a key-less entry system, the loop antenna device 51 is disposed in a door handle of a vehicle. In this case, since a magnetic field component has a plurality of axial components, the axial components cross in an orthogonal manner relative to the conductor such as the door parts. As shown in FIG. 7, the loop antenna device 51 is fixed such that the magnetic field component Hz crosses in an orthogonal manner relative to the conductor plate such as a door part in the vicinity of the conductor plate 57.
When the loop antenna device 51 is used for one part of a key-less entry system, the loop antenna device is disposed in spaced apart relation to the conductor plate 57 at a predetermined distance in order to secure an antenna characteristic. Otherwise, when the loop antenna device 51 is disposed in the vicinity of the conductor plate 57, the loop antenna device 51 is assembled by adjusting an antenna constant. In the condition shown in FIG. 7, when the power supply is oscillated, a radiation magnetic field Hz in a z-direction is generated on an inner portion of the coil. Then, as the magnetic field component xe2x88x92Hz is reflected by the conductor plate 57, the reflected magnetic field component (e.g., Hz) is denied by the magnetic field component xe2x88x92Hz generated by the coil. The loop antenna may be disposed apart from the conductor 57 in order to avoid the above-mentioned problem, however, if the loop antenna 51 is disposed apart from the conductor 57, it is necessary that the thickness of the door handle on a direction perpendicular to the vehicle door comes wider whereby the size of the vehicle door having the door handle becomes too large.
It is, therefore, one of the objects of the present invention to provide a loop antenna device without the forgoing drawbacks.
It is another object of the present invention to provide a loop antenna device having a radiated magnetic field generated by the coil of the loop antenna device when the loop antenna device is disposed in the vicinity of a conductor.
In order to attain the foregoing objects, a loop antenna device located close to a conductor includes an antenna for generating a magnetic field component perpendicular to the conductor, and an electromagnetic absorbing member disposed between the antenna and the conductor.
Further, a loop antenna device includes a first antenna having a first resonant circuit comprised by a first coil and a first condenser connected to the first coil, a second antenna including a second resonant circuit comprised by a second coil wound in a direction perpendicular to the wound direction of the first coil outside of the first antenna, a link coil wound in the same wound direction of the first coil and connected to the second coil, and a second condenser connected to the link coil, a case made of a conductor material accommodating the first antenna and the second antenna, and an electromagnetic wave absorbing member disposed between the case and at least one of the first coil and the second coil.